All bearings (tapered roller, cylindrical, ball, etc.) require some sort of retainer to hold them in place. Typically, the retainer must be very accurately located (especially in multiple bearing applications). This usually means use of adjustable nuts/bolts or a variety of shims under the retainer cap. When nuts/bolts are used, they are typically torqued to a specific value to generate the preload, which is most commonly needed with tapered roller bearings. Torquing bolts for a specific preload is inaccurate as many variables come into play such as thread pitch, lubrication, torque wrench accuracy, etc. When multiple nuts/bolts are used, an uneven loading situation may also occur. The use of shims allows one to obtain a preload or a gap. This provides a means to tightly bolt on the retainer, but the user typically must use a trial and error approach to determine the correct number/thickness of shims. This requires the end cap to be assembled onto the part, the resulting gap measured, and the correct thickness of the shims calculated. This must then be disassembled and reassembled with the shims now in place. A sufficient supply of shims must also be on hand at this time.
Due to the nature of tapered roller bearings and some ball type bearings, a radial load on the bearing will produce a separating force trying to push the bearings apart. If the separation is significant, it can reduce the contact area between the rollers (balls) and the races, severely shortening bearing life. To help combat this, some bearing manufacturers recommend preloading the bearings to help maintain the contact area. This preload typically has a range in which the bearing life remains fairly constant.
Cylindrical bearings and most ball bearings require a gap.